tg(a)-cos(a)/(1sin(a)=
=sin(a)cos(a)-cos(a) (1 sin(a)=
=(sin a *1-sin a *sin a - cos a *cos a) cos a *(1-
sin a-1) (cos a*1(1 sin a ))=-1/ cos a
tg(a)-cos(a)/(1sin(a)=
=sin(a)cos(a)-cos(a) (1 sin(a)=
=(sin a *1-sin a *sin a - cos a *cos a) cos a *(1-
sin a-1) (cos a*1(1 sin a ))=-1/ cos a